The ability of bone to form optimal structures to support loads and adapt structurally to changing loads is termed the "strain-adaptive remodeling response." The exact nature of the mechanical remodeling signal, or osteogenic stimulus, is not fully understood. However, several mechanical parameters have been proposed, including strain magnitude, frequency, and rate. It is thought that 1 to 2 Hz events during locomotion produce levels of strain on the order of 1000 to 3000 microstrain (or 0.1 to 0.3%), and are osteogenic in nature. These 1 to 2 Hz events are more fully described by Rubin C T and Lanyon L E, in the 1985 technical article "Regulation of Bone Mass by Mechanical Strain Magnitude," published in Calcified Tissue International, 37:411-417, which is herein incorporated by reference.
Further, it has been proposed that higher frequency events (15 to 25 Hz) of lower magnitude (100 to 250 microstrain), possibly associated with muscular contractions to maintain posture, are of importance in maintaining bone mass. These 15 to 25 Hz events are more fully described by Cowin, S C, in the 1997 report "Posture Load-Induced Bone Maintenance--A New Hypothesis" disclosed in NASA-funded Project #199-26-17-04, which is herein incorporated by reference. Removing this stimulus in environments such as those encountered during space flight will inhibit the process of bone deposition. It is well documented that bone loss is a physiologic effect of space flight. For example, this bone loss is more fully described by Rambaut P C, Smith M C Jr., Mack P B, Vogel J M, in the 1975 report "Skeletal Response" published in Biomedical Results of Apollo prepared by R S. Johnson, L. F. Dietlein, and C. A. Berry (eds.), (Document SP-377). Washington, D.C., pp. 303-322, which is herein incorporated by reference. In addition, this bone loss is also more fully described by Whedon, G. D., in the 1984 technical article "Disuse Osteoporosis: Physiological Aspects," published in Calcified Tissue International, 36, S146-S150, which is herein incorporated by reference. Thus, the accurate measurement of strain within this range of frequencies (1-25 Hz) and amplitudes (100-3000 microstrain (.mu..epsilon.)) is important for understanding the relationships between mechanical loading and bone remodeling.
A variety of methods exist for measuring animal bone strain in vitro and in vivo and are described, for example, in U.S. Pat. Nos. 5,456,724 and 5,695,496, both of which are herein incorporated by reference. Local strain in trabecular bone has been measured using optical devices to track the displacement of markers on individual trabeculae such as a method used by Michel M. C., et al in the technical article "Compressive Fatigue Behavior of Bovine Trabecular Bone," published in 1993--J. Biomechanics 26:453-463, and herein incorporated by reference.
Global strain has been measured in the tibia using metal-foil type strain gages adapted to intracortical pins that protrude from the skin as disclosed by Milgrom C., et al in "A Comparison of the Effect of Shoe Gear on Human Tibial Axial Strains Measured In Vivo" Abstract from ORS 43rd Annual Mtg., February 1997, and herein incorporated by reference.
The most common method has been to use metal foil type strain gages (for example, unstacked rosettes) bonded directly to the bone cortex. Surface-mounted gages are considered the standard for measuring cortical bone strain in vitro and in animal models in vivo. In humans, however, the use of surface-mounted gages is limited for several reasons; the compatibility of bonding material with living tissue is a problem (cyanoacrylate-based adhesives, which is the standard bonding material, are potentially carcinogenic), the level of invasiveness is high, and proper surface preparation, which is difficult to achieve on bone, is essential for obtaining reliable measurements. Among these methods, gages adapted to intracortical pins offer a less invasive and potentially more reliable way of obtaining in vivo strain data.
Surface-mounted strain gages when arranged in a rosette can provide principal strains and directions, and maximum shear strains within the plane of the gage. However, in the case of bone, where the moduli of cortical and trabecular bone vary greatly, surface strain gages mounted to the bone cortex are questionable indicators of global strain in trabecular bone. Further, if strains due to bending are to be calculated, surface gages must be mounted on opposing faces of the specimen, which is difficult to accomplish in vivo. Intracortical pins, which extend into the trabecular structure, can conceivably be used to follow global deformations and provide a measure of global strain across a section of bone in bending. Assumptions must be made that strain gradients are linear, and that the pins do not themselves deform. Also, as with unstacked strain gage rosettes, a uniform strain field is assumed in planes that contain the gage, as the gage necessarily covers a finite area of the test surface. Macroscopic (or average) strain is what is measured. For this reason, a smaller gage length is better. Since the porosity of the underlying cancellous bone restricts how small the gage length may be, a balance must be achieved between accurately capturing the strain field and sizing the gage length appropriately for a given specimen porosity. This is left to the user to determine for their particular. It is desired that means be provided for measuring in vivo strain encountered by the bones of a human and to do so with accuracy.